The present invention relates to separation processes and more particularly to an apparatus and method for separating solids from gas streams.
In the refining and chemical process industries, as well as in other processing industries, it is often necessary to remove finely divided solids from gaseous streams. The purpose of removing solids is to either protect the environment by removing such particles before the gases are emitted to the atmosphere, or to protect downstream equipment from the erosive action of the solids. In some cases the separated solids are returned to the process while in others the separated solids are collected for disposal outside of the process.
One widely used process in the petroleum refining industry that requires the removal of small particles is the fluid catalytic cracking (FCC) process for the cracking of heavy boiling gas oil streams to produce more valuable, lighter boiling products such as gasoline and lighter gases, primarily propanes and butanes. The FCC process utilizes a solid catalyst in powder form to facilitate the breaking of the carbon-carbon atomic bonds of the gas-oil feed to form smaller molecules that lie within the gasoline boiling range. In addition to the gasoline product, the process also produces substantial yields of lighter gases such as propanes and butanes that are recovered and converted to valuable products. Fluid catalytic cracking is the most widely used “conversion” process in petroleum refining and many millions of barrels per day of FCC capacity have been installed since the process inception in the early 1940's. As such, the FCC process is of great economic value and is typically the most profitable unit in a petroleum refinery in the United States as well as in most refineries around the world.
The catalyst used in the FCC process typically is a finely divided solid composed of mostly silica and alumina in both crystalline and amorphous form. The use of a powdered catalyst has been the key feature contributing to the success of the FCC process and has lead to the development of an entire area of process operations that has come to be known as “fluidization”. The finely divided powdered catalyst can be made to behave as a fluid when it is properly aerated or “fluidized” by means of air or another gas. The fluidized powder can be made to flow in lines and will establish a level within a vessel, as would a liquid. A fluidized powder will also generate a hydraulic pressure head proportional to the density and the depth of the mixture within a vessel or in a vertical standpipe as would a fluid. The powder can also be pneumatically transported by a gas stream when the gas has sufficient velocity. The ability of the powdered catalyst to flow between vessels has been of tremendous benefit in the development of a viable catalytic cracking process. Earlier attempts to use a fixed bed of catalyst pellets were largely handicapped by the need to regenerate the catalyst frequently to remove deposits of “coke” that are a by-product of cracking. The coke, which is mostly carbon with some hydrogen and sulfur, deactivates the catalyst and must be removed by means of a combustion step using air. In contrast, when a “fluidizable” catalyst is used, the catalyst can be continuously circulated between the reaction and regeneration vessels of a FCC unit so that there is no need for a cyclical process in order to accomplish the reaction and regeneration steps.
One major disadvantage in the use of a finely divided, powdered catalyst is that a portion of the catalyst remains suspended in the gas streams as the gas streams leave the reaction and regeneration vessels. The majority of the suspended catalyst solids must be removed before the gases exit the vessel. Otherwise, the catalyst will be so rapidly lost from the process that its use would be impractical.
The separation of the solid catalyst particles from the reaction products and from the regeneration flue gases in FCC processes is accomplished by means of centrifugal separation devices known as cyclones. The use of such centrifugal separators, or cyclones, for the purpose of removing solids from gaseous streams is well known. Cyclone separators work on the principal of creating a rapidly rotating vortex so as to induce a centrifugal force on the solids-bearing gas stream. The centrifugal force causes the entrained solids to concentrate on the wall of the cyclone body where they are slowed by friction and are thereby caused to separate from the gas stream.
In its simplest form, the typical cyclone separator consists of a vertical cylindrical body topped by a flat roof and having a conical lower section. The gas plus entrained solids enter at the upper part of the cylinder through a rectangular inlet duct that is tangential to the cylindrical body. The horizontal orientation of the tangential inlet causes the gas and solids to begin a rapid circular motion in the cyclone body that imparts a centrifugal force on the entrained solids. The solids move laterally through the rotating gas stream to the wall of the cyclone body. The rotating gas plus solids next enter the conical section of the lower cyclone where the tapered wall of the cone forces the downwardly moving solids and gas to move towards the center at the narrow end of the cone. At the end of the cone is a circular outlet duct to receive the separated solids. As the solids are separated from the gas and slowed by friction at the cylindrical wall of the cyclone body and in the tapered conical section, the action of gravity causes to solids to flow downwardly and into the solids outlet duct. The solids outlet duct generally extends a considerable length downward into the main vessel so that the recovered solids are returned well into the “active” reaction zone or “active” regeneration zone from which they were first entrained. These solids return ducts are termed “diplegs” and they are commonly sealed by means of a flapper valve or else they discharge directly into a bed of fluidized catalyst that acts as a seal. The flapper valve and/or bed seal serves as a simple check valve that lets the recovered solids flow out but prevents excessive backflow of gas from the vessel into the dipleg.
The rotating action of the entering gas and entrained solids in the cyclone body and lower cone of the cyclone separator forms a strong “vortex” that imparts the centrifugal force that causes the solids to move laterally to the wall. The gas outlet of the cyclone is a cylindrical tube that extends partway down into the cyclone body from the center of the cyclone roof. Therefore, to exit the cyclone, the rotating gas column must form another smaller, inner vortex that spirals upwardly to enter this gas outlet tube. Thus, in the typical cyclone separator there is an outer vortex in which the gas and solids spiral downwardly during which time the majority of the solids are forced to the wall by centrifugal action. The gas spiral must then undergo a reversal of direction and spiral back upwardly in the smaller inner vortex to enter the gas outlet tube at the roof of the cyclone. It is a common belief that there is a continuous flux of gas across the interface between the outer and inner vortices. The outer vortex loses gas to the inner vortex as it spirals downwardly and grows weaker and the inner vortex gains gas from the outer vortex as it spirals upwardly and grows stronger.
Cyclone separators have been developed over many years of use. The typical FCC reaction vessel will contain either two stages of cyclone separators or another type of primary separation device plus a single-stage cyclone system. The typical FCC regenerator will contain a two-stage cyclone system. Because of the large volume of gases that must be handled in the FCC reactor and regenerator, most units will have multiple sets of cyclones housed within the reaction and regeneration vessels. Typically the separation efficiency of an FCC cyclone system will exceed 99.99%. Still, some of the catalyst fines entrained in the cracked vapors leaving the reaction system or in the flue gases leaving the regeneration zone will not be captured in a two-stage cyclone system. Particulate material in the atmosphere, in particular very fine solids, can raise environmental issues. As a result, the emission of these fine solids to the atmosphere with the flue gas has come under increased scrutiny from environmental regulators during recent years. The presence of excessive catalyst fines in the reaction product can also cause problems for refiners by plugging downstream processing equipment or by making it difficult to produce a saleable product.
A typical FCC unit catalyst includes particles having a range of sizes from less than 20 microns to about 160 microns, with the average particle size being about 70 to 80 microns. The two-stage cyclone systems in most FCC units cannot retain catalyst fines smaller than about 20 microns for an extended period. These two-stage cyclone systems are generally housed within the reactor vessel (sometimes called a Disengager) or in the regenerator vessel. The catalyst captured by these systems is continuously returned directly to the FCC circulating inventory by means of the cyclone diplegs.
The flue gas leaving a FCC regenerator after two stages of cyclones will generally contain from 200 to 300 mg/Nm3 of catalyst fines. In many countries, the catalyst emissions are required to be below 50 mg/Nm3. As a result, a number of refiners have added an additional level of catalyst recovery to the FCC regenerator flue gas system to either meet the stringent requirements on solids emissions to the atmosphere or to protect downstream equipment. These additional levels of catalyst recovery consist of electrostatic precipitators (ESP), wet scrubbers, or another stage of centrifugal separators (cyclones). Electrostatic precipitators and scrubbers are used for the final cleanup of the flue gas before it goes to atmosphere and are capable of cleaning the flue gas to very low levels of solids, e.g., in the range of 10 to 20 mg/Nm3 of solids or less. However, precipitators and scrubbers are expensive to install and generally operate at relatively low pressures and/or temperatures relative to the flue gas exiting a FCC unit regenerator. Therefore, an electrostatic precipitator or scrubber cannot be used to protect an FCC unit flue gas power recovery system that operates at the unit pressure and relatively high temperature, which is generally in the range of 1200 to 1350 Deg. F. For this purpose refiners have employed an additional stage of high efficiency cyclones housed in a separate vessel downstream of the FCC regenerator vessel. Because these cyclones are in addition to the two stages of cyclones internal to the regenerator, they commonly go by the name of “third-stage separators” or TSS. The catalyst fines captured in these external flue gas separators are almost always discarded, as they are too small to be retained for very long in the circulating catalyst inventory.
Known third-stage separator systems employ multiple cyclone elements that are generally housed in a separate containment vessel. A third-stage system has the challenging task of trying to recover additional solids from a gas stream that has already been processed through two stages of cyclones. The particle size of the solids entering these systems is generally of less than about 30 microns and will sometimes be less than 20 microns. To recover additional solids from the flue gas exiting the FCC regenerator, a third-stage separator will generally use multiple cyclone elements that are smaller than the primary and secondary cyclones in the regenerator vessel. Smaller diameter cyclones are more efficient because, for a given gas inlet velocity, the centrifugal forces generated by the rotating gas are much higher due to the tighter radius and the solids have a much shorter path length to the wall.
A number of different cyclone designs are in operation in TSS systems. All use single-stage cyclonic elements. Some third-stage separators use conventional cyclone configurations similar to, but of generally of smaller diameter than, the primary and secondary cyclones in the regenerator vessel. Another commonly used third-stage system is that developed by Shell Oil Co. and referred to as a Shell Separator. This system uses small diameter cyclonic elements attached between an upper and a lower tube sheet. The cyclonic elements use an axial entry rather than a tangential entry as on a conventional cyclone. Swirl vanes are used in the axial gas entry to start the rotational motion of the gas. Although the cyclones are straight-sided with no cone section, they operate in a manner similar to a conventional cyclone in that the gas inlet and outlet are on the top side of the cyclone and the gas vortex must reverse direction in going from the inlet to the gas outlet. These cyclones are generally thought to operate with an efficiency of between 40 to 60% in removing the catalyst fines from the regenerator flue gas. The Shell separator is normally able to protect the downstream equipment, e.g. a flue gas turbo-expander, from the most damaging catalyst fines, i.e., those larger than about 10 microns. However, it is not able to consistently meet the more stringent emissions standards for flue gas discharged to the atmosphere.
Additional designs for cyclonic elements in TSS separators have recently been introduced that claim to be more efficient than the Shell Separator. U.S. Pat. No. 6,673,133 discloses a cyclone design that uses an axial inlet and is unidirectional in terms of the vortex flow, i.e., the vortex is formed at the top of the cyclone and exits the bottom and is not forced to change direction. In addition to the unidirectional flow, the improvement in efficiency also is said to be due to the configuration of the solids outlet. As is shown in FIG. 1b of U.S. Pat. No. 6,673,133, the solids outlets consist of multiple vertical slots located on the sides of the cyclone below the level of the gas outlet tube. The directional orientation of the slots and the pressure drop across the slots are also claimed to be important for increased efficiency. However, there are several disadvantages of this arrangement. More specifically, the slots are complex to construct, the slots may be subject to plugging, and there is a rigid connection between the gas outlet tube and the cyclone body.
It would be useful to develop an apparatus of simple construction and efficient operation that can achieve better separation than prior known fine particle separators such as third stage separators.